(a) Field of the Invention
The present invention relates to thermionic emission transistors, and more particularly it pertains to a new transistor which is operated by controlling the carriers which are caused to emit due to emission of thermoelectrons.
(b) Description of the Prior Art
Firstly, the characteristics and problems of conventional transistors will hereinafter be described to serve as the materials for explaining the operation principle, the characteristics and the superiority of the new transistor according to the present invention. The conventional three-terminal devices which are called "transistors" can be roughly grouped into the following three typical types.
One of them is a bipolar transistor which will hereinafter be referred to as BJT. This BJT has three terminals which are called "emitter", "base" and "collector". Another typical device is a "unipolar field effect transistor" which will hereinafter be referred to as FET. The three terminals of FET are "source", "gate" and "drain". The other typical device is a "static induction transistor" which will hereinafter be referred to as SIT. This latter SIT is a transistor which was developed relatively recently by Dr. Jun-ichi NISHIZAWA and was reported in IEEE Trans. On Electron Devices, vol. ED-22, pp 185-197, 1975. The characteristic and operational mechanism of this SIT was first reported in said literature. The three terminals of SIT are "source", "gate" and "drain" same as those of FET.
In these three kinds of conventional transistors, the region for supplying carriers, i.e., the emitter region in BJT and the source region in FET and SIT, is designed to have a high impurity concentration. This is because of the intention that carriers be supplied efficiency. The impurity concentration of the emitter or source region desirably is set as high as possible so long as other conditions permit, i.e., so long as crystal defects such as dislocation and strain caused by the introduction of impurities in the semiconductor material will not have any adverse effect on the characteristics of the device.
Control mechanism of the electric current which is allowed to flow in the main operation of these conventional transistors is as follows. That is, in a BJT, the amount of current flowing from emitter to collector is controlled by controlling the potential across the emitter and the base via the base resistance, by controlling the voltage applied to the base electrode serving as the control electrode of BJT. In an FET, on the other hand, the amount of the current flowing from source to drain is controlled by controlling the channel width on the source side by varying the potential gradient within the depletion layer due to the voltage applied to the gate (control) electrode and/or by varying the channel resistance, i.e., IR drop, due to the voltage applied to the drain. In a SIT, the amount of the flow of current is controlled by controlling the channel potential close to the source region by varying the potential profile within the depletion layer due to the voltage applied to the gate (control) electrode and/or through the potential profile within the depletion layer growing in the channel due to the voltage applied to the drain.
The primary feature of the transistor according to the present invention lies in that the current density can be made very large. When the current densities of these three types of conventional transistors are compared, the BJT is found to exhibit the largest current density per unit area of the device. Therefore, description will hereunder be made of the limit in terms of amount of current in BJT.
FIG. 1 shows schematically the construction of an npn type BJT. In the Figure, reference numeral 11 represents an n.sup.+ type emitter region, 12 a p type base region, and 13 an n type collector region. For the simplicity of explanation, the discussion will be made of a single dimensional structure. FIG. 2 shows the energy band diagram of the npn type BJT at the absence of applied voltage. In FIG. 2 the level indicated by a broken line represents the Fermi level E.sub.F. Since the emitter region has a high impurity concentration, the Fermi level is degenerated and is positioned to be in contact or in agreement with the bottom of the conduction band. FIG. 3 shows the energy band diagram of this BJT upon the application of a voltage, i.e., in an operative state. More specifically, FIG. 3 shows the energy band diagram under operating conditions wherein a forward voltage V.sub.be is applied across the emitter and the base and a reverse voltage V.sub.bc is applied across the base and the collector respectively. For convenience, the distribution n(x) of minority carriers injected into the base is schematically illustrated at the bottom of FIG. 3. Since a forward voltage V.sub.be is applied across the emitter and the base, and since a reverse voltage V.sub.bc is applied across the base and the collector, the minority carrier distribution n(x) within the base region decreases progressively from the emitter-base contact position (x=0) toward the base-collector contact (x=W.sub.b). On the other hand, when the density J of the injected electrons is small as compared with the acceptor density N.sub.A of the base region, and if the impurity concentration N.sub.A within the base region is assumed to be uniformly distributed, the current flows only in diffusion mechanism, and the current density J is given by: ##EQU1## wherein: q represents the unit electronic charge;
D.sub.nb represents the diffusion coefficient of electrons in the base region; and PA1 x represents the coordinate for position. PA1 P.sub.ne represents the hole density in the emitter region under thermal equilibrium and no applied voltage, and here P.sub.ne =ni.sup.2 /N.sub.D1 (wherein: N.sub.D1 represents the donor concentration of the n.sup.+ type emitter region); and PA1 L.sub.pe represents the diffusion distance of the holes in the emitter region. PA1 V.sub.bi represents the built-in voltage or diffusion potential. PA1 k represents the Boltzmann constant; PA1 T represents the temperature .degree.K.; PA1 N.sub.Aeb represents the impurity concentration of the base region on the emitter side; and PA1 N.sub.Abc represents the impurity concentration of the base region on the collector side. PA1 an emitter region formed with a semiconductor material having a first conductivity type and a high impurity concentration; PA1 a collector region formed with a semiconductor material having a first conductivity type and a high impurity concentration; and PA1 a base region made of a semiconductor material having a second conductivity type opposite to said first conductivity type and a high impurity concentration, that portion of said emitter region located adjacent to said base region having an energy band gap broader than that of the base region, that portion of said base region located adjacent to the emitter region having an impurity concentration of about 3.times.10.sup.18 cm.sup.-3 or more.
When the reverse voltage across the base and the collector is small, the electron density n(W.sub.b) at the junction of the base and collector regions is not so small. As V.sub.bc increases, n(W.sub.b) will become progressively smaller. More specifically, the density distribution of the electrons injected in the base region will become steeper with an increase in V.sub.bc. That is, as V.sub.bc grows larger, the minority carrier gradient dn(x)/dx becomes greater, and the current density J given by Formula (1) will increase progressively. However, if V.sub.bc exceeds a certain level, n(W.sub.b) will assume a negligibly small value. Unless the recombination of the minority carriers within the base region is extremely prominent, the Formula (1), in the negligible condition of n(W.sub.b), can be approximately rewritten to: ##EQU2## wherein: W.sub.b represents the base width. Ordinarily, n(0) is given by: ##EQU3## wherein: n.sub.pb herein mentioned represents the minority carrier concentration in the p type base region under thermal equilibrium and under no applied voltage, and is ni.sup.2 /N.sub.A (ni represents the intrinsic carrier concentration).
That is, as will be understood from Formulas (2) and (3), if the emitter-base forward voltage V.sub.be is determined, the amount n(0) of the minority carriers injected at x=0 is determined, since both the structure and the impurity concentrations of BJT have been set already. Accordingly, even when the value of V.sub.bc is increased beyond a certain level, it should be noted that the current density J which is given by Formula (2) will become constant. Needless to say, if a depletion layer expands due to an increase in V.sub.bc and if, as a result, W.sub.b is reduced, current density J will increase accordingly. As stated above, the current density J is subjected to limitation at a certain value thereof. In order to increase the value of current density J, it is desirable to reduce the value of base width W.sub.b. If the width W.sub.b of the base region is reduced progressively, the base resistance r.sub.bb of the base region located beneath the emitter electrode up to the base terminal will become large, and the signal voltage applied across the base and the emitter ceases to be uniform throughout the entire region of the emitter-base junction. Thus, the base width W.sub.b cannot be made very small. In order to suppress the increase of the base resistance r.sub.bb while reducing the base width W.sub.b, the impurity concentration N.sub.A of the base region has to be increased. However, if N.sub.A is increased too much, this will bring about a drop in the injection efficiency .gamma.. The injection efficiency .gamma. represents the ratio of the current caused by the carriers injected from the emitter region to the forward emitter-base current, and it is given by: ##EQU4## wherein: D.sub.pe represents the diffusion coefficient of the holes injected from the base region into the emitter region;
The condition in which Formula (4) is applicable in the instance in which the diffusion distance L.sub.pe is smaller than the thickness W.sub.e of the emitter region. If W.sub.e &lt;L.sub.pe, then L.sub.pe in Formula (4) is replaced by W.sub.e. As stated above, the minority carrier concentration n.sub.pb is expressed by: n.sub.pb =ni.sup.2 /N.sub.A. Accordingly, Formula (4) may be rewritten as: ##EQU5## As will be understood from Formula (5), when N.sub.A gains a level that is no longer negligible relative to the emitter donor concentration N.sub.D1, .gamma. becomes small, and the efficiency of operation of the transistor is denigrated. Usually, in a silicon BJT, if N.sub.D1 is selected to be in the order of 10.sup.20 -10.sup.21 cm.sup.-3, N.sub.A is selected at a value below the order of 10.sup.18 cm.sup.-3. It should be noted that it is because the width of the depletion layer produced between the base and emitter regions is sufficiently long as compared with the mean free path of the carriers that the number of the injected minority carriers at the emitter terminal of the base region at x=0 is given by Formula (3) or that the carrier density is expressed by Formula (1) or (2). The base-emitter depletion layer width W.sub.be, assuming that this junction is abrupt junction and N.sub.D1 &gt;&gt;N.sub.A, is given by: ##EQU6## wherein: .epsilon. represents the dielectric constant, i.e. permitivity and
As the forward voltage V.sub.be is increased, the depletion layer width decreases. For example, the width of the base-emitter depletion layer in a silicon device. With N.sub.A =1.times.10.sup.18 cm.sup..sup.-3, if V.sub.be =0 V, then W.sub.be .apprxeq.380 .ANG., and if V.sub.be =0.8 V, then, W.sub.be .apprxeq.200 .ANG.. Usually, the mean free path l of electrons within silicon at room temperature is considered to be about 50 .ANG. to about 100 .ANG.. In the case of GaAs, the path l is said to be a little longer than that. Accordingly, in the ordinary operating state of the device, W.sub.be is always greater than l. As such, from Formulas (2) and (3), the current density J of a known BJT having a constant impurity concentration of the base region is limited by: ##EQU7##
In the so-called drift transistor an impurity concentration distribution is provided in the base region in the direction from the emitter to collector, not only due to the injected minority carriers (which flow in the diffusion fashion through the base region as described above), but also by virtue of the effect of the electric field E(x) formed within the base region. The current density in such transistors is increased from Formula (7), and becomes ##EQU8## Here, if the impurity concentration distribution varies substantially exponentially, the electric field E(x) in the base region will substantially become: ##EQU9## wherein: .mu.n represents the electron mobility in the base region;
In such ordinary transistor having a uniform distribution of impurity concentration in the base region, the transport of carriers is dominated by diffusion. In a conventional BJT, even if arranged to be of a drift transistor structure (so that, for example, the width W.sub.b of the base region is made narrow such as 0.1 .mu.m and a drift field effect is applied) current density has been limited to about 10.sup.4 A/cm.sup.2 at maximum.
In each transistor of conventional BJT, FET and SIT, it is necessary to enhance the current density of the transistor in order to improve the high frequency characteristic and to provide a high-speed operation. That is, for high-speed operation and high-frequency characteristics it is necessary to arrange the device to cause the flow of as large a current as possible through as small an area as possible. However, even in a BJT which, among all conventional transistors, allows one to obtain the largest current density, there still is a limit for the current density which can be obtained. In order to obtain a very high current density, it is desirable that a potential barrier for controlling the injection of carriers be provided at a site located very close to the emitter or source region having a high impurity concentration serving as the region for supplying carriers, and that this potential barrier be efficiently controlled by a control electrode. However, if it is intended to efficiently control the height of the potential barrier by electrostatic control through a depletion layer in much the same way as in an SIT, the distance between the carrier-supplying region and the position of maximum height of the potential barrier must be substantially equal to the interval between the control electrodes. Therefore, if it is intended to reduce the distance between the carrier-supplying region and the position of maximum height of potential barrier, the interval between the control electrodes unavoidably becomes too narrow, and this is not desirable from the viewpoint of manufacture either. If it is intended to control the potential barrier through a resistance as in the case of BJT, this must be accomplished by an increase in the impurity concentration of the base region as discussed above. As a result, there is provided a transistor which has a lowered carrier injection effect and has a small current gain.